an1670 60 watts, gsm 900 mhz ldmos two-stage...

1MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION

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Prepared by: Jean–Jacques BounyPrincipal Staff EngineerMotorola Semiconductors S.A.Toulouse, France

INTRODUCTION

This application note demonstrates the feasibility of acomplete RF amplifier using Motorola LDMOS transistors inClass AB. The complete design requires standard piece partsand components only yet exhibits superior performance interms of gain, efficiency, power and ruggedness in a GSM 900MHz base station environment.

DESIGN RULES AND GOALS

The goal of the design is to provide an RF amplifier for GSM900 MHz base stations. The amplifier has to deliver an outputpower of 60 Watts CW (continuous wave) with an efficiency ashigh as possible, and a gain in the range of 30 dB. The finaltransistor chosen is the MRF184, a second generationLDMOS transistor from Motorola. The MRF184 is able todeliver 60 W with more than 10 dB gain in a linearconfiguration. The power supply is 26 V (typically 24 to 28 Vfor base stations). The driver stage has to deliver a maximumpower in the range of 4 W, but in order to have very goodlinearity (concentrating the non–linearities in the final stageincreases the overall efficiency) the MRF6522–10, a 10 WSMD transistor was chosen. This device is also a secondgeneration LDMOS transistor from Motorola. Matchingnetworks are done on an epoxy substrate (GI180 from

POLYCLAD, εr = 4.0), and the PCB is screwed on the baseplate using standard 2.5 mm screws. SMD 0805 componentsare used, and the “ACCUP” series from AVX are used for “highQ” capacitors. All metalized holes in the PCB have a 0.5 mmdiameter, including the hole area used for the grounding andthe cooling of the driver.

IMPEDANCE MEASUREMENTS

The first step in such a design is the determination of thesource and load impedances for the transistors, optimized forbest performances in a GSM application. The source and loadimpedance values are provided in Table 1. Note that valuesare quite different from the “S” parameters for the MRF184 andMRF6522–10. “S” parameters should not be used for thedesign of a high power Class AB amplifier and are onlysuitable as a starting point to verify maximum available gainand stability over a specified bandwidth.

It can be seen in the Smith chart for Figure 1 that theimpedances of the two devices are not very dispersed, whichmeans that it will be quite easy to build a wideband amplifier.This will allow mass production of the amplifier with no or verylimited tuning.

Smith is a registered trademark of Analog Instruments Co.Libra is a trademark of Hewlett–Packard, Inc.

Order this documentby AN1670/D

"SEMICONDUCTOR APPLICATION NOTE

Motorola, Inc. 1998

REV 0

2 MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION

fMHz S11 S22

900

920

940

0.66 + j4.71

0.61 + j4.89

0.64 + j4.79

2.41 + j2.91

2.32 + j2.94

2.26 + j3.02

960

980 0.59 + j5.03

0.58 + j4.97 2.23 + j3.05

2.22 + j3.27

ZinOhms

ZoutOhms

0.60 – j0.93

0.57 – j0.82

0.59 – j0.88

1.48 – j0.82

1.50 – j0.77

1.62 – j0.71

0.55 – j0.66

0.56 – j0.73 1.79 – j0.60

1.82 – j0.49

MRF184 (26 V, 70 Watts)

fMHz S11 S22

900

920

940

2.65 – j2.44

2.60 – j1.99

2.55 – j2.25

7.08 – j9.21

6.85 – j8.96

6.66 – j8.78

960

980 2.44 – j1.58

2.47 – j1.81 6.52 – j8.51

6.32 – j8.29

ZinOhms

ZoutOhms

2.20 – j2.20

2.14 – j1.77

2.18 – j1.89

6.05 – j8.50

5.76 – j8.09

4.88 – j7.15

2.05 – j1.32

2.10 – j1.65 4.53 – j6.36

3.54 – j4.97

MRF6522–10 (26 V, 12 Watts)

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Figure 1. Input and Output Impedances

Table 1. Input and Output Impedances


FINAL STAGE DESIGN

Although the critical task in a multistage amplifier design isthe interstage matching, it is recommended to focus on thefinal stage matching (50 Ω input, 50 Ω output) as a startingpoint. The above impedances have been used in a simulator(Libra from HP–EEsof) to define the input and output

matching networks. The results are shown in Figure 2. At theoutput, some capacitors have been split in two in order toavoid heating problems in the component itself. It can be seenthat the matching networks are quite simple and use standardvalues of components.

Figure 2. Final Stage PCB

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Figure 3. Final Stage Electrical Schematic

C1 5.6 pF (AVX ACCUP)C2, C3 15 pF (AVX ACCUP)C4, C5 10 pF (AVX ACCUP)C6 2.7 pF (AVX ACCUP)

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C7 2.2 pF (AVX ACCUP)C8, C9 470 pF (NPO)R1 3.9 kΩR2 1 kΩ


Figure 2 shows a view of the PCB with the positioning of thecomponents (solder stencil). A fine tuning of the amplifier canbe done in a specific application by adjusting the length of theparallel inductances (0.5 mm printed lines). In Figure 4, onecan see the complete amplifier board including bias circuit. Asolder mask has been added on the PCB in order to ensuregood positioning of the RF components. The amplifier wasconstructed with only a resistor bridge, since the purpose ofthis paper was not to concentrate on the different types of biascircuits. Of course, any type of bias circuit can be implementedwith the same RF circuit, thermally compensated or not.

Figure 4. Final Stage Complete Amplifier

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FINAL STAGE MEASUREMENTS

A complete set of measurements was taken on the finalstage. Figures 5 to 8 show measurements taken with anetwork analyzer.

Figure 5. Power Gain, S11 = f (Frequency)

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Figure 7. Power Gain, S11 = f (Pin @ 925 MHz)

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Figure 8. Power Gain, S11 = f (Pin @ 960 MHz)

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Figures 5 and 6 show gain and input return losses versusfrequency (GSM band and wide band), and Figures 7 and 8show gain and input return losses versus input power at bothends of the band (up to 1 dB compression point). Input powerscale is relative to the input power necessary to have 1 dB gaincompression.


At 1 dB compression point, the following results areobtained:

925 MHz / 26 V / 35°C base–plate66 W / 13.2 dB gain / 58.6% drain efficiency

960 MHz / 26 V / 35°C base–plate64 W / 13.1 dB gain / 55.6% drain efficiency

As shown in Figures 9 to 11, some additionalmeasurements have also been done to show the behavior ofthe amplifier versus temperature, power supply andsaturation level.

Figure 9. Power Gain, Efficiency = f (Pout)

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Figure 10. Pout @ 1 dB Compression = f (VDD)

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Figure 11. Pout @ 1 dB Compression = f (Temperature)

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As seen in Figure 12, reverse intermodulation has also beenmeasured with two different levels of spurious in order to covermost of the applications. Spacing between the main tone andthe spurious is 400 kHz, and reverse intermodulation ismeasured by reference to the level of the main carrier.

Figure 12. Reverse IMD = f (Pout)

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DRIVER STAGE DESIGN

The driver stage was also matched as a “stand alone” inorder to assess what the performance would be as the finalstage of a microcell (low power) base station. Figures 13 to 15show the design of the amplifier, and Figures 16 and 17 showthe results obtained versus frequency.

At 1 dB compression, the following results are obtained:

925 MHz / 26 V / 35°C base–plate15 W / 18.5 dB gain / 62% efficiency

960 MHz / 26 V / 35°C base–plate12 W / 18.0 dB gain / 57% efficiency

Figure 13. Driver Stage Electrical Schematic

C1 8.2 pF (AVX ACCUP)C2 18 pF (AVX ACCUP)C3 10 pF (AVX ACCUP)C4 12 pF (AVX ACCUP)

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C5, C6 470 pF (NPO)R1 3.9 kΩR2 1 kΩ

Figure 14. Driver Stage PCB

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Figure 15. Driver Stage Complete Amplifier

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As seen in Figure 16, the driver stage and the finalstage have a bandwidth much larger than the GSM band(925 MHz–960 MHz), and could also be used for theDAMPS 900.

TWO–STAGE DESIGN

For the design of the two–stage amplifier, the outputmatching network of the final stage and the input matchingnetwork of the driver stage are used. An interstage networkwill then be designed that goes directly from the outputimpedance of the driver to the input impedance of the finalstage without crossing 50 Ω. This will simplify the totalline–up layout and save space and components. Figures 18to 20 show the circuit diagram and the layout of the amplifier.Figures 21 and 22 show the results obtained versusfrequency.


Figure 18. Two–Stage Electrical Schematic

C1, C3 8.2 pF (AVX ACCUP)C2 18 pF (AVX ACCUP)C4, C6, C7, C8 10 pF (AVX ACCUP)C5 12 pF (AVX ACCUP)C9 2.7 pF (AVX ACCUP)

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C10 2.2 pF (AVX ACCUP)C11, C12, C13, C14 470 pF (NPO)R1, R3 3.9 kΩR2, R4 1 kΩR5 15 Ω

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Figure 19. Two–Stage PCB

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Figure 20. Two–Stage Complete Amplifier

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At 1 dB compression, the following results are obtained:

925 MHz / 26 V / 35°C base–plate65 W / 30.0 dB gain / 53.3% efficiency

960 MHz / 26 V / 35°C base–plate63 W / 30.4 dB gain / 51.4% efficiency

It can be seen that the performances of the two–stageamplifier are similar to those obtained for the final stage alone.This is because the driver has been a little bit oversized, andit does not bring any gain compression on the total line–up.This increases the power capability of the complete amplifierand also increases the overall efficiency.

MANUFACTURABILITY ANALYSIS

In order to verify the potential reliability andmanufacturability of the design, a risk analysis (placement ofthe components and of the RF transistor) and thermalmeasurements have been done on the two–stage amplifier.Thermal measurements have been done with an infraredmicroscope (Computherm III from Barnes), and provide thepicture in Figure 23 where one can see the temperature of thedifferent matching elements. The amplifier was on a heat sinkand temperature was regulated to obtain 70°C at thetemperature test point (in the brass base plate).


Figure 23. Two–Stage IR Scanning

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The risk analysis has been done on the positioning of theinput and output capacitors and on the positioning of the RFtransistor. Capacitors were moved by ±0.1 mm on each side(window within the solder mask), and the RF transistor wasmoved by ±0.2 mm within the groove in the drain–gate axis.Results given below show extreme variations of power,efficiency and gain.

Input Matching:

Pout @ 1 dB Eff @ 1 dB Gps @ 1 dB(W) (%) (dB)

± 0.5 ±1 ±0.15

Output Matching:


± 0.65 ±0.6 ±0.2

Device Positioning:


± 0.5 ±1 ±0.1

Bad grounding of the PCB near the transistor also has beensimulated without significative effect.

ALTERNATIVE DESIGN

There are many ways to optimize the performance of anRF amplifier. One way would be to select a PCB material withimproved thermal properties, such as a 0.51 mm thickRO4003 from ROGERS, and solder it down to a thin metalbase plate. When making these changes, as described inFigures 24 and 25, the temperature of the matching elementsis significatively reduced; the output parallel line goes from127°C down to 98°C, and the maximum temperature ofcapacitors C8 and C9 goes from 118°C down to 85°C with70°C at the temperature test point.


Figure 24. Alternative Two–Stage ElectricalSchematic

C1 8.2 pF (AVX ACCUP)C2 10 pF (AVX ACCUP)C3 8.2 pF (AVX ACCUP)C4, C5, C6, C7 10 pF (AVX ACCUP)C8 3.3 pF (AVX ACCUP)

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C9 2.7 pF (AVX ACCUP)C10, C11, C12, C13 470 pF (NPO)R1, R3 3.9 kΩR2, R4 1 kΩR5 22 Ω

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Figure 25. Alternative Two–Stage PCB

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CONCLUSION

It has been shown that it was possible to build a simplecompact amplifier for GSM 900 MHz base stations by usingonly two stages of Motorola LDMOS transistors.Performances are good even on an epoxy substrate (morethan 60 W, 30 dB gain and 50% total efficiency). The designcan be easily reproduced for the driver or final stage only, orfor the two–stage amplifier. Two solutions are proposed

using two types of PCB material, with two ways of mountingthe PCB. The choice between these solutions will dependon the actual design rules used for the design of the radio,as well as the manufacturing capabilities. A complete set ofmeasurements shows that this application is well suited forthe GSM specification.

Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regardingthe suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, andspecifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motoroladata sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights ofothers. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or otherapplications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injuryor death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorolaand its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney feesarising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges thatMotorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an EqualOpportunity/Affirmative Action Employer.

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